The instant invention relates to circuits for generating accurate reference frequencies.
The invention applies in particular to the generation of reference frequencies for modulators or demodulators of frequency modulated signals.
The invention has been implemented in connection with modulation and demodulation circuits for luminance and chrominance TV signals, and, in particular, for recording these signals in a videotape recorder. For these circuits, a plurality of accurate frequencies corresponding to standard values are generated for the various existing transmission systems (PAL, SECAM, NTSC).
Those accurate frequencies are conventionally generated by means of quartz oscillators with a specific quartz crystal for each of the frequencies to be generated. However, this method is expensive, mainly because the quartz crystal is not incorporated into an integrated circuit wherein the reference frequencies are used.
Another method consists of using only one quartz and logic circuits for counting, frequency dividing, and digital-analog converting, etc, for generating a plurality of different frequencies. For example, the crystal oscillation frequency can be divided for defining a calibration time duration, large enough with respect to the crystal frequency and a voltage controlled oscillator (VCO) provides a signal having the desired frequency. The VCO provides the desired frequency, different from the quartz frequency by counting the number of oscillation periods emitted by this quartz during the calibration time duration defined by the crystal oscillator and the dividers. The period number is compared with a programmed adjustable value which defines the various desired output frequencies. The result of this comparison is used for incrementing a counter, the output of which controls a digital-analog converter. The converter drives the oscillator for adjusting the output frequency thereof at a value that is accordingly associated with the programmed adjustable value.
This type of solution uses a large area of an integrated circuit due to the large number of complex circuits to be used (counters, dividers, converters).